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United States Patent |
5,738,472
|
Roopnarine
,   et al.
|
April 14, 1998
|
Modified locking thread form for fastener
Abstract
A threaded fastener has a standard part with a standard thread form
characterized by thread walls with a standard included angle, and a
modified part complementary to the standard part having a modified thread
form characterized by thread walls which are symmetrically inclined with a
modified included angle that is different from the standard included angle
of the standard part's thread walls, such that the threads of one part
make pre-loaded edge contact with the thread walls of the other part. The
thread form of the modified part can have an included angle that is
greater, less, or compound as compared to the included angle of the
standard part. The standard part may be a bolt and the modified part a
nut, or vice versa. The modified thread form holds securely even under
large vibrational forces, it permits bi-directional use of standard mating
threads, is impervious to the build up of tolerances and can be
manufactured with a wider range of tolerances without loss of
functionality, and distributes loading stresses (per thread) in a manner
that decreases the possibility of single thread failure.
Inventors:
|
Roopnarine; (New York, NY);
Vranish; John D. (Crofton, MD)
|
Assignee:
|
Honeybee Robotics Inc. (New York, NY)
|
Appl. No.:
|
733345 |
Filed:
|
October 17, 1996 |
Current U.S. Class: |
411/309; 411/366.3; 411/411 |
Intern'l Class: |
F16B 035/04; F16B 039/30 |
Field of Search: |
411/308,309,310,311,324,411,366
|
References Cited
U.S. Patent Documents
271852 | Feb., 1883 | Hull | 411/311.
|
1016897 | Feb., 1912 | Ryan | 411/308.
|
1451484 | Apr., 1923 | Woodward | 411/311.
|
3133578 | May., 1964 | Moskovitz | 411/309.
|
3426820 | Feb., 1969 | Phipard, Jr. | 411/310.
|
Primary Examiner: Wilson; Neill R.
Attorney, Agent or Firm: Chong; Leighton K.
Goverment Interests
The invention described herein as made with U.S. Government support under
contract NAS5-32,904 awarded by NASA. The U.S. Government has certain
rights in this invention.
Claims
We claim:
1. A threaded fastener comprising:
a first fastener part having a uniformly repeating pattern of threads,
wherein said threads have a standard thread form characterized by thread
walls which are symmetrically inclined with a predetermined standard
included angle and by a cross-sectional profile having a first thread
crest portion and a first thread root portion; and
a second fastener part complementary to the first fastener part having a
uniformly repeating pattern of corresponding threads, wherein said
corresponding threads have a modified thread form characterized by thread
walls which are symmetrically inclined with a modified single-included
angle that is different from the standard included angle of the first
fastener part's thread walls and by a cross-sectional profile having a
second thread crest portion and a second thread root portion;
wherein said modified single-inclined angle and crest and root profile of
the thread walls of said second fastener part are formed relative to those
of said first fastener part such that the crest portions of one fastener
part have a clearance with the root portions of the other fastener part
prior to pre-loading, and the crest portions of the one fastener part then
elastically deform into wedged contact with the root portions of the other
fastener part upon preloading of the complementary fastener parts.
2. A threaded fastener according to claim 1, wherein the second fastener
part has thread walls with a modified included angle greater than the
standard included angle of the first fastener part.
3. A threaded fastener according to claim 1, wherein the second fastener
part has thread walls with a modified included angle less than the
standard included angle of the first fastener part.
4. A threaded fastener according to claim 1, wherein the first fastener
part has thread walls with a standard included angle of approximately
60.degree., and the second fastener part has thread walls with a modified
included angle greater than 60.degree..
5. A threaded fastener according to claim 1, wherein the first fastener
part has thread walls with a standard included angle of approximately
60.degree., and the second fastener part has thread walls with a modified
included angle less than 60.degree..
6. A threaded fastener according to claim 1, wherein the first fastener
part is a standard bolt and the second fastener part is a nut having
modified internal threads.
7. A threaded fastener according to claim 1, wherein the first fastener
part is a standard nut and the second fastener part is a bolt having
modified external threads.
8. A threaded fastener according to claim 1 wherein the first fastener part
is a standard bolt for a Class 3 fit for a 1/2"--20 screw system, and the
second fastener part is a nut having modified internal threads.
9. A threaded fastener according to claim 1, wherein the first fastener
part is a standard bolt for a Class 3 fit for a #10--32 screw system, and
the second fastener part is a nut having modified internal threads.
10. A modified fastener part, for use with a complementary standard
fastener part having a uniformly repeating pattern of threads of a
standard thread form characterized by thread walls which are symmetrically
inclined with a predetermined standard included angle and by a
cross-sectional profile having a standard thread crest portion and a
standard thread root portion, said modified fastener part having a
uniformly repeating pattern of corresponding threads, wherein said
corresponding threads have a modified thread form characterized by thread
walls which are symmetrically inclined with a modified single-included
angle that is different from the standard included angle of the standard
fastener part's thread walls and by a cross-sectional profile having a
second thread crest portion and a second thread root portion;
wherein said modified single-inclined angle and crest and root profile of
the thread walls of said modified fastener part are formed relative to
those of the standard fastener part such that the crest portions of one
fastener part have a clearance with the root portions of the other
fastener part prior to pre-loading, and the crest portions of the one
fastener part elastically deform into wedged contact with the root
portions of the other fastener part upon preloading of the complementary
parts.
11. A modified fastener part according to claim 10, having thread walls
with a modified included angle greater than the standard included angle of
the standard part.
12. A modified fastener part according to claim 10, having thread walls
with a modified included angle less than the standard included angle of
the standard part.
13. A modified fastener part according to claim 10, wherein the standard
part has thread walls with a standard included angle of approximately
60.degree., and the modified fastener part has thread walls with a
modified included angle greater than 60.degree..
14. A modified fastener part according to claim 10, wherein the standard
part has thread walls with a standard included angle of approximately
60.degree., and the modified fastener part has thread walls with a
modified included angle less than 60.degree..
15. A modified fastener part according to claim 10, wherein the standard
part is a bolt and the modified part is a nut having modified internal
threads.
16. A modified fastener part according to claim 10, wherein the standard
part is a nut and the modified part is a bolt having modified external
threads.
17. A modified fastener part according to claim 10, wherein the standard
part is a bolt for a Class 3 fit for a 1/2"--20 screw system, and the
modified fastener part is a nut having modified internal threads.
18. A modified fastener part according to claim 10, wherein the standard
part is a bolt for a Class 3 fit for a #10--32 screw system, and the
modified fastener part is a nut having modified internal threads.
Description
This patent application claims the priority of U.S. Provisional application
Ser. No. 60/005,574 filed on Oct. 18, 1995.
FIELD OF THE INVENTION
This invention generally relates to a fastener for bolted joints or
fixtures, and particularly to a modified locking thread form that can
increase resistance to loosening under vibrational loads.
BACKGROUND ART
The prior art consists of unidirectional thread forms, conventional torque
fasteners, and other secondary fastening methods, e.g., epoxy and
lockwashers for preventing standard bolts and nuts from vibrating loose.
Such methods usually employ the deformation of a secondary element to
prevent the slight movements that cause fasteners to work loose.
Elastic deformation of fasteners usually results in a one-time use for the
elements and may further inhibit servicing in hostile environments such as
in space. Unidirectional thread forms limit the designs that can be used
in that they dictate the manner in which a particular joint must be
designed in order to make effective use of the locking feature. Secondary
locking elements, such as lockwashers, score the surfaces which are
clamped or fastened together, and epoxying fasteners to prevent relative
motion can also result in unserviceable joints. Excessive preloading is
necessary in most instances to input enough energy into the joint so as to
increase resistance to vibrational loads.
SUMMARY OF THE INVENTION
In accordance with the present invention, a modified locking thread form is
provided that has the advantage of being axially reversible, can be an
external or internal thread, is compatible and designed to be used with a
standard mating thread form, can be manufactured cheaply and effectively
as nuts, bolts or taps, and can be reused many times without any loss in
its functionality. Further, the invention distributes loads in an
effective manner in which there is a significant improvement in the
force/thread ratio. The invention is expected to be used wherever
vibration resistance is critical or needed to prevent loss of preload and
the loosening of joints.
In accordance with the present invention, a threaded fastener has a first
fastener part having a uniformly repeating pattern of threads, wherein
said threads have a standard thread form characterized by thread walls
which are symmetrically inclined with a predetermined standard included
angle, and a second fastener part complementary to the first fastener part
having a uniformly repeating pattern of corresponding threads, wherein
said corresponding threads have a modified thread form characterized by
thread walls which are symmetrically inclined with a modified included
angle that is different from the standard included angle of the first
fastener part's thread walls such that the threads of one of the
complementary parts make pre-loaded edge contact with the thread walls of
the other of the complementary parts.
In different versions, the modified thread form has thread walls with an
included angle that is greater, less, or compound as compared to the
included angle of the thread walls of the standard fastener part. The
standard part may be a bolt and the modified part a nut, or vice versa.
The modified thread form allows edge contact to be made between the threads
of the complementary parts such that they deform bitingly into each other
and hold securely even under large vibrational forces. The modification of
a standard thread form permits the use of standard mating threads with the
invention and simultaneously provide high resistance to vibration
loosening. Symmetrical modification of the standard thread form permits
the bidirectional use of the thread form. The modified thread form is
impervious to the build up of tolerances and can be manufactured with a
wider range of tolerances without loss of functionality. It also
distributes loading stresses (per thread) in a manner that decreases the
possibility of single thread failure even when the threaded material is
soft.
Other objects, features and advantages of the present invention are
described in detail below in conjunction with the drawings, as follows:
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1A shows a nut having a first version of the modified thread form in
accordance with the invention, and FIG. 1B shows a profile view of the
thread form.
FIG. 2 is a cross-sectional view of the first version applied to a modified
internal thread form for a nut.
FIG. 3 is a cross-sectional view of the first version applied to a modified
external thread form for a bolt.
FIGS. 4A and 4B show a comparison of cross-sectional views of the first
version as compared to a second version applied to a modified internal
thread form for a nut for a 1/2"--20 screw system.
FIGS. 5A and 5B show a comparison of cross-sectional views of the first
version as compared to a second version applied to a modified internal
thread form for a nut for a #10--32 screw system.
FIG. 6A shows a nut having a third version of the modified thread form in
accordance with the invention, and FIG. 6B shows a profile view of the
thread form.
DETAILED DESCRIPTION OF THE INVENTION
A first version of the present invention is shown in the accompanying FIGS.
1A and 1B representing a modification of the standard thread form. In FIG.
1A, a nut fastener 10 for use with a corresponding threaded bolt has a
central aperture 11 and an internal threading 12 through the thickness of
the nut. As shown in FIG. 1B, the threading 12 consists of a uniformly
spaced pattern of thread grooves each having a modified cross-sectional
shape which accomplishes the purposes of the invention. The thread
cross-section has a flat top land 12a which makes contact in the
complementary groove of the corresponding bolt, a slightly inclined first
wall 12b, a more steeply inclined second wall 12c, and a flat bottom land
12d. As an example, the thread grooves can have an outside radius (to the
bottom land) of 0.250 inch from the centerline of the nut aperture, an
inside radius of 0.226 inch, for a depth of 0.024 inch. The slightly
inclined first wall 12b can extend half the depth, e.g., 0.12 inch, and be
inclined at an angle of about 10.degree. to the plane direction of the
nut. The more steeply inclined second wall 12b can extend the other half
of the depth, e.g., 0.12 inch, and be inclined at an angle of about
30.degree. to the plane direction. The combined first wall thread profile
is 20.degree., and the combined second wall thread profile is 60.degree..
The total width of the groove between the edges of consecutive top lands
is 0.032 inch.
Standard fasteners lose their preload because of the existence of
accumulated tolerances (that is, clearance) between the threads of a nut
and a bolt during manufacturing. This phenomenon results in the formation
of air gaps between the mating threads when a joint is bolted together.
Under cyclic loading, the cumulative effect of these air gaps is to allow
microscopic motion between the internal and external threads such that the
bolt and nut eventually work loose along the axis. Once the preloading
force is lost the joint failure can be catastrophic. This invention
eliminates the effect of loose tolerance fits in a joint by guaranteeing a
close fit after the system (i.e. joint) is preloaded.
FIG. 2 shows the effect of preloading an internally threaded system
represented by the version of FIG. 1 (compound angle system). In FIG. 2, a
standard bolt 20 is secured through two bodies 22 and 23 by the nut 10
having the modified internal thread form 12. The internal thread form has
the second thread profile of 60.degree. maintained at the thread root to
provide clearance for free-running a bolt, and the first thread profile
inclined at 20.degree. (included angle) at the crest of the thread. When
the joint is preloaded, the approximate point of contact between the
threads is on the flank near the root of the external thread (standard
form) and the crest of the 10.degree. flank of the internal thread, as
opposed to in the conventional thread form the 30.degree. flank contacting
another 30.degree. flank.
The modified thread form allows edge contact to be made between the threads
of the complementary parts such that they deform bitingly into each other
and hold securely even under large vibrational forces. The modified thread
form is impervious to the build up of tolerances (that results in the
existence of clearance between standard mating threads) and can,
therefore, be manufactured with a wider range of tolerances without loss
of functionality. It also distributes loading stresses (per thread) in a
manner that decreases the possibility of single thread failure even when
the threaded material is soft (e.g. aluminum).
The modification of a standard thread form permits the bidirectional use of
standard mating threads with the invention and simultaneously provide high
resistance to vibration loosening. The bidirectionality of the thread form
allows holes in a component to be tapped from either direction (as is the
case with standard threads) thereby providing flexibility in design.
In FIG. 3, the modified thread form of the first version is instead
provided on the external threads of the bolt 30 while a standard nut 32 is
used. When the modified thread form of the bolt 30 is preloaded against a
standard internal thread form of the nut 32, the crest of the modified
thread (at the included angle of 20.degree.) comes into contact with the
standard thread of the nut at approximately its root. The same effect of a
securely locking is obtained as in the modified internal thread form in
FIG. 2.
As compared to the first version illustrated in side-by-side fashion in
FIG. 4a, a second verson of the invention having a single angle of thread
wall is shown in FIG. 4b for a Class 3 fit with a standard 1/2"--20 bolt.
In this version, the width of the root or bottom 42c of the internal
thread of the nut 40 is slightly larger than the top of standard external
thread form of the bolt 41, and the total included angle between the
thread flanks 42b is made smaller, e.g., 50.degree., as compared to the
flanks of the standard external thread form having an angle of 60.degree..
Thus, the crests or top lands 42a of the modified internal thread form 40
make contact with the flanks near the root of the standard external thread
form 41. The intersection points of the modified thread form may be made
slightly rounded without reducing the intended effect.
FIGS. 5A and 5B show a similar comparison of modified thread form profiles
for a Class 3 fit with a #10--32 bolt.
Standard practice in the fastener manufacturing industry recognizes three
classes of fit, namely, Class 1, Class 2, and Class 3, which specify
various tolerances (and consequently, clearances) between male and female
thread forms. In this case, the disclosed example is manufactured to mate
with a standard male or female thread form with a tolerance specified by a
Class 3 fit. The invention is not limited to a Class 3 fit and in fact can
perform with the same resistance to loosening with other classes.
A third version (reverse single angle), which is the reverse of the second
version described above, provides the same advantages of the previously
described versions. As shown in FIG. 6, the nut 60 has a central aperture
61 with an internal thread form 62 having an included angle of 70.degree.
for its flanks 62b which is larger than the standard 60.degree. angle of
the standard external thread form. The standard external thread has a
smaller width than the modified internal thread at its crest 62a but is
larger at the base (the internal thread root 62c is smaller). Preloading
of this third version provides area contact between the crest of the
standard screw thread and the flank near the root of the modified nut
thread thereby removing clearance and providing effective locking.
The root width and included angle are selected such that there is enough
clearance for free running a bolt into the nut. The thread profiles that
result from these modifications provide significantly better load
distribution since the loading contact points change to contact areas
under elastic deformation of the threads.
The tolerances needed to manufacture modified screw thread systems of the
three versions are selected such that, when used with a standard bolt or
screw (Class 3A), each will provide superior resistance to vibration
loosening. Furthermore, each version will provide vibration resistance
with Class 1 and 2 fits and are not dependent on a Class 3 fit for this
effect. The preloading process, in all cases, effectively removes the
loose fit between the mating threads and prevents relative motion between
them. A high resistance to loosening under vibration therefore result
regardless of Class fits.
As described above, the modified thread form has a tip angle of 20.degree.
(included) in the first version (compound angle system). In the second
version (single angle), the included angle is less than the standard
60.degree.. In the third version (reverse single angle), the included
angle is greater than 60.degree.. All of these internal thread
modifications are completely reversible in that they can be used in the
same way as external thread modifications. Again, the modification of
contact areas will occur near the crest of the thread for the first
version, and at the thread root and the included angle in the second and
third versions. It is also possible that any included angle (less than the
standard 60.degree. for the second version and greater than 60.degree. for
the third version) can be used to provide the same effect in both
versions. An optimal angle can be chosen which will be dependent on ease
of manufacturability, best resistance to vibrational loads and
distribution of loads (force/thread).
The invention improves upon the prior art by providing ease of
manufacturability, a symmetrical profile which ensures its bidirectional
feature, reversibility (i.e. internal or external forms are possible),
high reusability without loss of functionality and high resistance to
vibrational loads as indicated by the thread forms' (each version) ability
to pass the MIL-STD-1312A, Method 7 test in initial testing. The forms of
the threads are such that they are impervious to the build up of
tolerances (that results in the existence of clearance between standard
mating threads) and can, therefore, be manufactured with a wider range of
tolerances without loss of functionality. The bidirectionality of the
thread forms allow holes in a component to be tapped from either direction
(as is the case with standard threads) thereby providing flexibility in
design. Finally, the invention distributes loading stresses (per thread)
in a manner that decreases the possibility of single thread failure even
when the threaded material is soft (e.g. aluminum).
Although the invention has been described with reference to certain
preferred embodiments, it will be appreciated that many other variations
and modifications thereof may be devised in accordance with the principles
disclosed herein. The invention, including the described embodiments and
all variations and modifications thereof within the scope and spirit of
the invention, is defined in the following claims.
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